1. Technical Field
This invention relates generally to a system for controlling a variable-reluctance (VR) motor, and more particularly, to an apparatus for external inductance sensing for controlling VR motor commutation.
2. Discussion of The Related Art
Variable-reluctance (VR), or as they are alternatively known, switched-reluctance (SR) machines have been the subject of increased investigation due to their many advantages, which makes it suitable for use in a wide variety of situations. A VR machine operates on the basis of varying reluctance in its several magnetic circuits. In particular, such machines are generally doubly salient motors--that is, they have teeth or poles on both the stator and the rotor. The stator teeth have windings which form machine phases of the motor. In a common configuration, stator windings on diametrically opposite poles are connected in series to form one machine phase.
When a stator phase is energized, the closest rotor pole pair is attracted towards the stator pole pair having the energized stator winding, thus minimizing the reluctance of the magnetic path. By energizing consecutive stator windings (i.e., machine phases) in succession, in a cyclical fashion, it is possible to develop torque, and thus rotation of the rotor in either a clockwise, or counter-clockwise direction.
As further background, the inductance of a stator winding associated with a stator pole pair varies as a function of rotor position. Specifically, the inductance varies from a lower level when a rotor pole is unaligned with a corresponding stator pole, where it rises to an upper or maximum level when the rotor pole and stator pole are in alignment. Thus, when the rotor pole rotates and sweeps past a stator pole, the inductance of the stator winding varies through lower-upper-lower inductance levels. This inductance-versus-rotor position characteristic is particularly relevant for controlled operation of the motor. Specifically, current flowing through the stator winding must be switched on prior to (i.e., advanced), and maintained during the rising inductance period to develop positive torque. Since positive phase current during the decreasing inductance interval produces a negative or breaking torque, the phase current must be switched off before this interval occurs to avoid negative torque. Accordingly, rotor position sensing is an integral part of a closed-loop variable-reluctance motor drive system so as to appropriately control torque generation.
The prior art has taken two fundamentally opposing approaches in determining rotor position: direct methods, and indirect methods. Direct methods include the use of direct rotor position sensors, such as optical encoders and Hall effect devices, which are commonly used in closed-loop motor drives for the purpose of phase current commutation. However, sensors of this type increase the cost of the drive system, and are not sufficiently rugged, (i.e., are relatively unreliable) in automotive applications.
Indirect methods were investigated, partially, due to the shortcomings of the above-mentioned direct techniques. In one indirect method, advanced control theory techniques are used, such as an observer-based state variable model, to estimate rotor position using operating parameters such as phase current, voltage, or inductance of deenerigized stator windings. However, one disadvantage of these types of methods is that they require an expensive processing device, such as a microprocessor, to acquire and evaluate the numerous samples needed to determine the rotor position. Further, performance is generally poor with these methods at the outer limits of the motor operating range (i.e., very low speed, and high speed conditions).
Accordingly, there is a need to provide an improved apparatus for commutation of a variable-reluctance machine that minimizes or eliminates one or more of the problems as set forth above.